1. Field of the Invention
The present invention relates to an electric power steering device for applying a steerage assisting power to the steering mechanism of a vehicle by an electric motor. Moreover, the present invention relates to a control method utilized for controlling said power steering device.
2. Description of the Related Art
In the related art, there has been used the electric power steering device for applying the steerage assisting power to the steering mechanism by driving the electric motor in accordance with the steering torque applied by the driver to the handle (or the steering wheel). This electric power steering device is provided with a torque sensor for detecting the steering torque to be applied to the handle or operation means for the steerage, to output a torque detection signal indicating the steering torque, so that the target value of an electric current to be fed to the electric motor is set on the basis of the torque detection signal coming from the torque sensor. And, a command value to be fed to the drive means of the electric motor is generated by a proportional integration controller on the basis of a deviation between the target value and a current to flow actually through the electric motor. The drive means of the electric motor includes: a PWM signal generating circuit for generating a pulse width modulation signal (or a PWM signal) at a duty ratio according to that command value; and a motor drive circuit constructed of power transistors to be turned ON/OFF according to the duty ratio of the PWM signal. The drive means applies a voltage according to the duty ratio, i.e., a voltage according to the command value, to the electric motor. The current to be fed to the electric motor by that voltage application is detected by a current detecting circuit, so that the difference between the aforementioned target value and the detected value is used as a deviation for generating the aforementioned command value. Thus in the electric power steering device, the feedback control is so made that the current at the target value set on the basis of the steering torque indicated by the torque detection signal from the torque sensor may flow through the electric motor.
The values of the proportional gain and the integration gain (as will be called the “PI gain”) of the proportional integration controller are desired to be higher for enhancing the responsibility of the entire system. If the value of the PI gain is excessively high, however, the system is liable to instabilities in the vicinity of the natural oscillation frequency of the mechanical line, e.g., in the vicinity of 10 to 25 Hz. In the related art, therefore, there is provided a phase compensator for stabilizing the system not by setting the value of the PI gain excessively high but by sacrificing the responsibility of the entire system and for improving the phase characteristics in a practical frequency band. Specifically, the torque detection signal from the torque sensor is given to a phase compensator so that its phase is advanced by the phase compensator thereby to improve the responsibility of the entire system in the practical frequency band. A new low-pass filter is added to the frequency band, in which the gain characteristics are enhanced according to that construction.
As described above, the target value of the current to be fed to the electric motor is determined according to the steering torque compensated by the phase compensator. More specifically, an inertia compensating current value for suppressing the influences of the moment of inertia of the electric motor, a damping control current value for improving the convergence of the steering wheel, a return control current value for improving the operability at the returning time of the steering wheel, and so on are added to the current value determined according to the steering torque compensated by the phase compensator, thereby to determine the target value of the current to be fed to the electric motor. Generally, the inertia compensating current value is determined according to the differential value of the steering torque. Such construction is disclosed in JP-A-2002-249063, for example.
In the electric power steering device thus far described, however, the inertia compensating current value is determined on the basis of the differential value of the steering torque, as compensated by the phase compensator so that it is not always identical to the result determined on the actual steering torque. Therefore, it may occur that the control result expected cannot be obtained. A similar problem also arises in case the various compensation control such as the aforementioned damping control or the various judgments are done on the basis of the steering torque compensated by the phase compensator.
In the aforementioned electric power steering device, moreover, the target value of the current to be fed to the electric motor is frequently determined on the basis of a prepared assist table (i.e. assist map). However, this assist table contains a dead zone and characteristic break points. In the neighborhood control regions, therefore, the expected characteristics, as designed, may not be attained. In case this assist table is altered, moreover, it is necessary to redesign the parameters of the phase compensator corresponding to the assist table altered.